From conservation of momentum and energy, it turns out that if two objects bump into each other and end up moving at the same speed, energy must have been lost. I know the formulas and calculations, but what physically happens at the contact site? Why cannot one object just push the other one, until it moves as fast as the first object, and then stop pushing? Why does it have to cause heat? How does it cause the heat? Another related question I have, is that why is momentum conserved, if energy is not, ie if some of the force doesn't get applied to the other body, but instead gets converted to heat, why is it that momentum is conserved? Conservation of momentum comes from action-reaction, if you push one object, you get pushed in the other direction with the same force, but if some of the force through distance (energy) is applied to heating, why is momentum still conserved?